High pressure X-ray study on CPDT-TCNQ anion radical salts

Isostructural anion radical salts Me4X(CPDT-TCNQ)2 (X=N, P, As) have extremely one-dimensional electronic system. We investigated their high pressure properties from X-ray diffraction measurements. The superlattice reflections were observed at room temperature above 7.9, 12.1 and 9.5 kbar on the Me4N, Me4P and Me4As salts, respectively. They are relevant to the stepwise increase of resistivity under pressure. The origins of the transitions are thought to be the 2kF CDW for the Me4N and Me4P salts, and the 4kF CDW for the Me4As salt. In addition, the 4kF-to-2kF transition is observed at 10.4 kbar on the Me4As salt.